Ni—Ti alloys with a nickel content comprised between 50 and 52 atom % pertain to the category of thermoelastic materials (also known in the field as Nitinol. Shape Memory Alloys, “smart” materials, etc), and according to the finishing process they undergo (e.g., training, shape setting, education, etc), they may exhibit a shape memory effect or a superelastic behavior. Details of suitable processes and characteristics of these alloys are widely known in the art and may be found in C. M. Wayman, “Shape Memory Alloys” MRS Bulletin, April 1993, 49-56, M. Nishida et al., “Precipitation Processes in Near-Equiatimic TiNi Shave Memory Alloys”. Metallurgical Transactions A, Vol 17A, September, 1986, 1505-1515 and H. Hosoda et al., “Martensitic transformation temperatures and mechanical properties of ternary NiTi alloys with off stoichiometric compositions”, Intermetallics, 6 (1998), 291-301, all of which are herein incorporated by reference in their entirety.
These alloys are employed in a variety of applications. By way of example and not of limitation, in industrial applications, shape memory wires are used in actuators as a replacement for small motors. Further applications for such thermoelastic materials include the medical field, where they are used for stents, guidewires, orthopedic devices, surgical tools, orthodontic devices, eyeglass frames, thermal and electrical actuators, etc.
Independently from the final shape of the Ni—Ti thermoelastic device, that can for example be wire or tube or sheet or bar based, the manufacturing process includes a cutting phase from a longer metallic piece, obtained from a semi-finished product resulting from an alloy melting process. The most common forms for the semi-finished products are long tubes, wires, rods, bars, sheets.
The behavior of these Ni—Ti alloys is strongly dependent on their composition. The presence of one or more additional elements may result in new properties and/or significantly alter the characteristic and behavior of the alloy. The importance of the purity of the Ni—Ti alloy is addressed in US Pub. App. US2006/0037672, incorporated herein by reference in its entirety.
U.S. Pat. No. 4,337,900 discloses use of Ni—Ti alloys with an additional amount of copper ranging from 1.5 to 9 atom % to improve workability and machinability.
Another ternary modification of Ni—Ti alloys with reference to superleastic alloys is described in PCT patent publication WO2002063375, where a wide compositional range is described. In particular, the substituent, chosen from Cu, Fe, Nb, V, Mo, Co, Ta, Cr and Mn, may vary between 1% and 25 atom %.
European patent EP 0465836 discloses addition of carbon and optional small metal amounts. The carbon amount is comprised between 0.25 and 5 atom %. The optionally added metals are comprised between 0.25 and 2 atom % and are chosen from V, Cr, Fe, Nb, Ta, W, and Al.
Improved corrosion and wear resistant Ni—Ti alloys are disclosed in U.S. Pat. No. 3,660,082, where such effect is achieved substituting nickel with one or more metals chosen from Fe, Mo, Co, and Cr, while Ti is substituted with Zr. The nickel substitution range is 1-50 atom % and the titanium substitution range is 0-10 atom %.
The addition of a rare earth element in order to get a radiopaque alloy is disclosed in PCT publication WO2008/030517 where additions of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Tn, Pa and U may be made in an atom percentage range between 0.1 and 15.
Japanese patent application JP 59028548 discloses alloys, where nickel or titanium atoms are substituted with no more than 1 atom % of one or more elements chosen from V, Cr, Mn, Fe, Co, Cu, Zr, Nb, Mo, Ta and noble metals.
Japanese patent application JP 63235444 describes Ni—Ti—Al alloys having good phase transformation at low temperature, where Al is up to 2 atom %, and where up to 1 atom % of one or more elements chosen from V, Cr, Mn, Co, Zr, Nb, Mo, Ru, Ta and W may be present.
JP 60026648 describes an annealing and cold rolling finishing process for Ni—Ti alloys containing up to 3 atom % of one or more elements chosen from V, Cr, Mn, Fe, Co, Cu, Zr, Nb, Mo, Pd, Ag, Ru, Ta and W.
All these references teach the addition or alternatively the substitution (decreasing the amount of titanium or nickel in proportion to the amount of the additional element) of one or mm element to Ni—Ti alloys to modify their properties.